The conventional methods for producing condensation polymers involve reacting molecules containing complimentary functional groups in an organic solvent. For example, the production of a polyurethane traditionally comprises the reaction of a polyisocyanate with an active hydrogen species in the presence of an organic solvent, such as acetone.
One specific type of polyurethane synthesis is the preparation of waterborne polyurethanes which typically contain an internal emulsifier in the polyurethane chain. Such internal emulsifiers are typically active hydrogen bearing compounds that contain ionic or latent ionic functionality, or some other water soluble moiety. An important class of internal emulsifier useful for the preparation of polyurethane dispersions is dihydroxy carboxylic acids which is exemplified by dimethylol propionic acid (DMPA). These polymers which show hydrophilic ionic sites between predominantly hydrophobic chain segments spontaneously form stable dispersion in water without the influence of shear forces and in the absence of dispersants.
The aqueous polyurethane dispersion can be obtained by first reacting a diisocyanate or mixture of diisocyanates with a polyol, alone or with other active hydrogen compounds, and a suitable amount of a polyol containing an ionic or latent ionic functionality in an organic solvent. The ratio of isocyanate functionality to active hydrogen groups is chosen such that a high molecular weight polyurethane is obtained. The latent ionic functionality present in the polyurethane chain is then neutralized with a suitable base to provide ionic functionality. Water is added to the solution of the ionic polyurethane in organic solvent until water becomes the continuous phase. A fully aqueous dispersion can be obtained by distilling the organic solvent from the resulting mixture. See Rossthauser, J. W. et al., "Waterborne Polyurethanes", Advanced Urethanes Sci Technology, Vol. 10, 1987, pp. 121-162.
Alternatively, a prepolymer with free isocyanate groups can be prepared by reacting a diisocyanate or mixture of diisocyanates with a polyol, alone or with other active hydrogen compounds and a suitable amount of a polyol containing an ionic or latent ionic functionality employing an excess of isocyanate functionality relative to active hydrogen species. After neutralization of the prepolymer to develop the ionic functionality, the prepolymer is dispersed rapidly in water. The molecular weight of the prepolymer is increased by chain extension in water usually with a di- or polyfunctional amine or diol. Typically, it is necessary to use some organic cosolvent in the prepolymer synthesis in order to control the viscosity of the prepolymer. A fully aqueous dispersion is obtained by distilling the cosolvent from the dispersion, or the solvent is left in the dispersion. Supra.
The presence of the organic cosolvent is a disadvantage in both cases. Removal of this solvent requires time and energy and is commercially uninteresting. Simply leaving the solvent in the dispersion is often unacceptable as the object of preparing water-borne polyurethanes is to eliminate organic solvent at the point of product use.
Aqueous media have replaced solvents in synthetic production of systems such as polyurethanes, however, it is well known that in the reaction between the polyisocyanate and polyol to produce the polyurethane that a third reaction comes into play with the reaction of the polyisocyanate with water. Water hydrolyzes isocyanate groups to yield the amines via the unstable carbamic acid with the evolution of carbon dioxide. Supra.
Densified, particularly supercritical, fluids offer a desirable alternative to both solvent and aqueous based synthetic methods. Densified carbon dioxide provides a non-toxic, inexpensive, recyclable and environmentally acceptable solvent and heretofore has not been utilized in the synthesis of condensation polymers.
Supercritical fluids, such as supercritical carbon dioxide, have been used in a process to produce condensation polymers, such as polyurethanes, polyepoxides, polyesters and polycarbonates, etc. by inhibiting the chemical reactions between the functional groups of the organic materials. See EP 506 041 (Union Carbide). The publication describes the inhibition of a reaction between a polyol and a diisocyanate for up to ten hours under supercritical conditions. When the conditions are reduced to atmospheric conditions, the two materials react instantly. The publication describes the selection of one or more compounds which are not soluble in supercritical fluids to separate the compounds effectively and prevent the condensation reaction from occuring until the supercritical fluid conditions are no longer present.
Carbon dioxide has also been used at atmospheric pressures as a blowing agent to form flexible polyurethane foams. See U.S. Pat. No. 5,200,435 (The Dow Chemical Company). Although it was found desirable to replace chlorofluorocarbon blowing agents with carbon dioxide for environmental reasons, in general the use of carbon dioxide generating blowing agents results in flexible polyurethane foams with poor physical properties such as tear strength, tension strength and elongation.
There thus exists a need for an improved synthetic method for the production of condensation polymers, such as polyurethanes, which produces a final product free of solvent residue in high yields and which is both environmentally friendly and cost effective.